Modeling the boundaries of growth of Salmonella typhimurium in broth as a function of temperature, water activity, and pH

The growth limits of a mixture of five strains of Salmonella Typhimurium in tryptic soy broth were examined at different environmental conditions. The response of the pathogen was monitored in a total of 350 combination treatments of temperature (10 to 35degreesC), pH (3.76 to 6.44), and water activity (a(w), 0.913 to 0.990) for 62 days. No growth/growth (turbidity) data were modeled by logistic polynomial regression. The concordance index of the logistic model was 99.8%, indicating a good fit to the observed data. The minimum pH and a(w) values that permitted growth were 3.94 and 0.942, respectively, and occurred in the temperature range of 25 to 35degreesC. At temperatures below this range, the minimum pH and a(w) allowing growth increased as the temperature decreased. The results showed an abrupt change in the probability of growth close to the boundary with minor changes of the environmental factors. The probabilities predicted by the model were compared with published data on the actual response of Salmonella Typhimurium or other salmonellae serotypes in 50 cases of food products, including salad dressing, mayonnaise, meat, cheese, vegetables, and fruits. The model predicted successfully the response of the pathogen in 90% of the tested cases. The results of the study indicated that the developed model predicts satisfactorily the growth/no growth interface of Salmonella Typhimurium in foods and can provide useful quantitative data for the development of safer food products and processes.